Applying colors to artwork is a common Adobe Illustrator
task, and one that requires some knowledge of color models and color
modes. When applying color to artwork, keep in mind the final medium
in which the artwork will be published, so that you can use the
correct color model and color definitions. Experimenting and applying
color is easy using the feature-rich Swatches panel, Color Guide panel,
and Edit Colors/Recolor Artwork dialog box in Illustrator.

Color
models describe the colors we see and work with in digital graphics.
Each color model, such as RGB, CMYK, or HSB, represents a different
method for describing and classifying color. Color models use numeric
values to represent the visible spectrum of color. A color
space is a variant of a color model and has a specific gamut (range)
of colors. For example, within the RGB color model are a number
of color spaces: Adobe® RGB, sRGB, and Apple® RGB. While each of
these color spaces defines color using the same three axes (R, G,
and B), their gamuts are different.

About colors in digital graphics

We
use color models to describe the colors we see and work with in
digital graphics. Each color model, such as RGB, CMYK, or HSB, represents
a different method for describing and classifying color. Color models
use numeric values to represent the visible spectrum of color. A color
space is a variant of a color model and has a specific gamut (range)
of colors. For example, within the RGB color model are a number
of color spaces: Adobe® RGB, sRGB, and Apple® RGB. While each of
these color spaces defines color using the same three axes (R, G,
and B), their gamuts are different.

When you work with the colors in a graphic, you are actually
adjusting numerical values in the file. It’s easy to think of a
number as a color, but these numerical values are not absolute colors
in themselves—they only have a color meaning within the color space
of the device that is producing the color.

Because each device has its own color space, it can reproduce
colors only in its gamut. When an image moves from one device to
another, image colors may change because each device interprets
the RGB or CMYK values according to its own color space. For example,
it is impossible for all the colors viewed on a monitor to be identically
matched in a print from a desktop printer. A printer operates in
a CMYK color space, and a monitor operates in an RGB color space. Their
gamuts are different. Some colors produced by inks cannot be displayed
on a monitor, and some colors that can be displayed on a monitor
cannot be reproduced using inks on paper.

Even though it is impossible to perfectly match all colors on
different devices, you can use color management to ensure that most
colors are the same or similar enough so they appear consistent.

RGB, CMYK, HSB, and Lab color models

RGB

A large percentage of the visible spectrum
can be represented by mixing red, green, and blue (RGB) colored
light in various proportions and intensities. Where the colors overlap,
they create cyan, magenta, and yellow.

RGB colors are
called additive colors because you create white by
adding R, G, and B together—that is, all light is reflected back
to the eye. Additive colors are used for lighting, television, and
computer monitors. Your monitor, for example, creates color by emitting
light through red, green, and blue phosphors.

Additive colors (RGB)

A. Red B. Green C. Blue

You can work with color values using the RGB color mode, which
is based on the RGB color model. In RGB mode, each of the RGB components
can use a value ranging from 0 (black) to 255 (white). For example,
a bright red color might have an R value of 246, a G value of 20,
and a B value of 50. When the values of all three components are
equal, the result is a shade of gray. When the value of all components
is 255, the result is pure white; when all components have values
of 0, the result is pure black.

Illustrator
also includes a modified RGB color mode called Web Safe RGB,
which includes only those RGB colors that are appropriate for use
on the web.

CMYK

Whereas
the RGB model depends on a light source to create color, the CMYK model
is based on the light-absorbing quality of ink printed on paper.
As white light strikes translucent inks, a portion of the
spectrum is absorbed. Color that is not absorbed is reflected back
to your eye.

Combining
pure cyan (C), magenta (M), and yellow (Y) pigments would result
in black by absorbing, or subtracting, all colors. For this reason
they are called subtractive colors. Black (K) ink is
added for better shadow density. (The letter K came
into use because black is the “key” color for registering other
colors, and because the letter B also stands for blue.) Combining
these inks to reproduce color is called four-color process
printing.

Subtractive colors (CMYK)

A. Cyan B. Magenta C. Yellow D. Black

You can work with color values using the CMYK color mode, which
is based on the CMYK color model. In CMYK mode, each of the CMYK
process inks can use a value ranging from 0 to 100%. The lightest
colors are assigned small percentages of process ink colors; darker
colors have higher percentage values. For example, a bright red
might contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In CMYK
objects, low ink percentages are closer to white, and high ink percentages are
closer to black.

Use CMYK when preparing a document to be printed using process
inks.

HSB

Based on the human perception of color,
the HSB model describes three fundamental characteristics of color:

Hue

Color reflected from
or transmitted through an object. It is measured as a location on
the standard color wheel, expressed as a degree between 0° and 360°.
In common use, hue is identified by the name of the color, such
as red, orange, or green.

Saturation

Strength
or purity of the color (sometimes called chroma). Saturation
represents the amount of gray in proportion to the hue, measured
as a percentage from 0% (gray) to 100% (fully saturated). On the
standard color wheel, saturation increases from the center to the
edge.

Brightness

Relative lightness or darkness of the color,
usually measured as a percentage from 0% (black) to 100% (white).

HSB color model

A. Hue B. Saturation C. Brightness

Lab

The CIE Lab color model is based on the
human perception of color. It is one of several color models produced
by the Commission Internationale d’Eclairage (CIE), an organization
dedicated to creating standards for all aspects of light.

The numeric values in Lab describe all the colors that a person
with normal vision sees. Because Lab describes how a color looks
rather than how much of a particular colorant is needed for a device
(such as a monitor, desktop printer, or digital camera) to produce
colors, Lab is considered to be a device-independent color
model. Color management systems use Lab as a color reference to predictably
transform a color from one color space to another color space.

In Illustrator, you can use the Lab model to create, display,
and output spot color swatches. However, you cannot create documents
in Lab mode.

Grayscale

Grayscale uses tints of black to represent
an object. Every grayscale object has a brightness value ranging
from 0% (white) to 100% (black). Images produced using black-and-white
or grayscale scanners are typically displayed in grayscale.

Grayscale also lets you convert color artwork to high-quality
black-and-white artwork. In this case, Adobe Illustrator discards
all color information in the original artwork; the gray levels (shades)
of the converted objects represent the luminosity of the original
objects.

When you convert grayscale objects to RGB, the color values for
each object are assigned that object’s previous gray value. You
can also convert a grayscale object to a CMYK object.

Color spaces and gamuts

A color
space is a range of colors in the visible spectrum. A color
space can also be a variant of a color model. Adobe
RGB, Apple RGB, and sRGB are examples of different color spaces
based on the same color model.

Gamuts of different color spaces

A. Visual gamut B. RGB color
space C. CMYK color space

The range of color encompassed by a color space is called a gamut.
The different devices (computer monitor, scanner, desktop printer,
printing press, digital camera) throughout your workflow operate
within different color spaces and each with different gamuts. Some
colors within the gamut of your computer monitor are not within
the gamut of your inkjet printer, and vice versa. When a color cannot
be produced on a device, it’s considered to be outside the color space
of that particular device. In other words, the color is out
of gamut.

About spot and process colors

You
can designate colors as either spot or process color types, which
correspond to the two main ink types used in commercial printing.
In the Swatches panel, you can identify the color type of a color
using icons that appear next to the name of the color.

When applying color to paths and frames, keep in mind the final
medium in which the artwork will be published, so that you apply
color using the most appropriate color mode.

Note:

If your color workflow involves transferring
documents among devices, you may want to use a color-management
system (CMS) to help maintain and regulate colors throughout the
process.

About spot colors

A spot color is
a special premixed ink that is used instead of, or in addition to, process
inks, and that requires its own printing plate on a printing press.
Use spot color when few colors are specified and color accuracy
is critical. Spot color inks can accurately reproduce colors that
are outside the gamut of process colors. However, the exact appearance
of the printed spot color is determined by the combination of the
ink as mixed by the commercial printer and the paper it’s printed
on, not by color values you specify or by color management. When
you specify spot color values, you’re describing the simulated appearance
of the color for your monitor and composite printer only (subject
to the gamut limitations of those devices).

Keep the following guidelines in mind when specifying a spot
color:

For best results in printed documents, specify a spot
color from a color-matching system supported by your commercial
printer. Several color-matching system libraries are included with
the software.

Minimize the number of spot colors you use. Each spot color
you create will generate an additional spot color printing plate
for a printing press, increasing your printing costs. If you think
you might require more than four colors, consider printing your
document using process colors.

If an object contains
spot colors and overlaps another object containing transparency,
undesirable results may occur when exporting to EPS format, when converting
spot colors to process colors using the Print dialog box,
or when creating color separations in an application other
than Illustrator or InDesign. For best results, use the Flattener
Preview or the Separations Preview to soft proof the effects of
flattening transparency before printing. In addition, you can convert
the spot colors to process colors by using the Ink Manager in InDesign
before printing or exporting.

You can use a spot color printing plate to apply a varnish
over areas of a process color job. In this case, your print job
would use a total of five inks—four process inks and one spot varnish.

About process colors

A process color is
printed using a combination of the four standard process inks: cyan,
magenta, yellow, and black (CMYK). Use process colors when a job
requires so many colors that using individual spot inks would be
expensive or impractical, as when printing color photographs.

Keep the following guidelines in mind when specifying a process
color:

For best results in a high-quality printed document,
specify process colors using CMYK values printed in process color
reference charts, such as those available from a commercial printer.

The final color values of a process color are its values
in CMYK, so if you specify a process color using RGB (or LAB, in
InDesign), those color values will be converted to CMYK when you
print color separations. These conversions differ based on your
color-management settings and document profile.

Don’t specify a process color based on how it looks on your
monitor, unless you are sure you have set up a color-management
system properly, and you understand its limitations for previewing
color.

Avoid using process colors in documents intended for online
viewing only, because CMYK has a smaller color gamut than that of
a typical monitor.

Illustrator and InDesign let you specify a process color
as either global or non-global. In Illustrator, global process colors
remain linked to a swatch in the Swatches panel, so that if you
modify the swatch of a global process color, all objects using that
color are updated. Non-global process colors do not automatically
update throughout the document when the color is edited. Process
colors are non-global by default. In InDesign, when you apply a
swatch to objects, the swatch is automatically applied as a global
process color. Non-global swatches are unnamed colors, which you
can edit in the Color panel.

Note:

Global and non-global process colors only affect how a particular
color is applied to objects, never how colors separate or behave
when you move them between applications.

Using spot and process colors together

Sometimes it’s
practical to use process and spot inks in the same job. For example,
you might use one spot ink to print the exact color of a company
logo on the same pages of an annual report where photographs are
reproduced using process color. You can also use a spot color printing
plate to apply a varnish over areas of a process color job. In both
cases, your print job would use a total of five inks—four process
inks and one spot ink or varnish.

In InDesign, you can mix process and spot colors together to
create mixed ink colors.

Comparing colors in InDesign and
Illustrator

Adobe InDesign and Adobe Illustrator use slightly different
methods for applying named colors. Illustrator lets you specify
a named color as either global or nonglobal, and InDesign treats
all unnamed colors as nonglobal, process colors.

The
InDesign equivalents to global colors are swatches.
Swatches make it easier to modify color schemes without having to
locate and adjust each individual object. This is especially useful
in standardized, production-driven documents like magazines. Because
InDesign colors are linked to swatches in the Swatches panel, any
change to a swatch affects all objects to which a color is applied.

The InDesign equivalents to nonglobal swatches are unnamed colors.
Unnamed colors do not appear in the Swatches panel, and they do
not automatically update throughout the document when the color
is edited in the Color panel. You can, however, add an unnamed color
to the Swatches panel later.

Named and unnamed colors only affect how a particular color updates
in your document, never how colors separate or behave when you move
them between applications.